The present invention is directed to start-up circuits for voltage regulators, in particular band-gap regulators. Band-gap regulators are well known in the art. They provide a tightly regulated voltage with a low temperature coefficient. In order to prevent the regulator from changing the output voltage as the supply voltage varies, (Power Supply Rejection Ratio or PSRR), the regulator is normally biased via a constant current source. To achieve even a greater degree of PSRR, the current source reference current is normally derived from the band-gap regulator itself. This bias scheme though results in two stable states. The first is the normal regulating mode and the second a condition where the regulator does not "start-up" and remains off.
During the initial application of power to a band-gap regulator, some method must be provided to ensure that the regulator starts up. Normally a separate circuit is used to provide the initial start up current.
FIG. 1 is a schematic of a typical band-gap regulator and start-up circuit of the prior art. The band-gap regulator 10 has a terminal 12 for receiving start-up current, an output terminal 14 for providing the regulated voltage, an output terminal 16 for providing the regulated band-gap voltage, a ground pin 18 and a terminal 20 for receiving the source voltage. The start-up circuit of the prior art includes an input voltage 22 connected in series with a resistor 24 and a pair of diodes 26 and 28. The input voltage is typically about 3 volts. The base of a transistor 30 is connected between the pair of diodes and the resistor 24. The collector of the transistor 30 is connected to the start-up current terminal 12 of the band gap regulator and the emitter of the transistor 30 is connected to the band-gap voltage terminal 16. The voltage at the base of the transistor 30 is the sum of the forward voltages of the diodes, typically 1.2 V.
In operation, as power is supplied to the circuit, the regulated input voltage 22 supplies current to transistor 30. The diodes 26 and 28 clamp the base of the transistor 30 to the sum of the two forward voltages. The current pulled out of the start-up current terminal 12 by transistor 30 forces the bandgap regulator circuit 10 to start operating. In the regulator shown in FIG. 1, current mirror 11 begins conducting. As the regulator approaches proper operating conditions, the voltage on the emitter of transistor 30 begins to rise to the band-gap voltage which in this application is about 1.2 V. When the band-gap voltage gets high enough, the transistor 30 becomes reverse biased and stops conducting.
In practicing the prior art start-up circuit, it is necessary to ensure that the clamp voltage at the base of transistor 30 is sufficiently high in magnitude to ensure that the transistor 30 conducts enough current to start up the regulator. If the voltage at the base of transistor 30 is too low, it is possible that the regulator will reach a stable state somewhere between zero volts and the desired regulated voltage. Since it is necessary to provide two diodes 26 and 28 to ensure a high enough voltage, the start-up circuit has a net negative temperature coefficient. The temperature coefficients of diode 26 and transistor 30 cancel each other out, but the negative temperature coefficient of diode 28 then causes the voltage at the base of transistor 30 to rise at low temperatures. Since under regulation, the emitter of transistor 30 is held at a relatively stable voltage, it is possible that at low temperatures transistor 30 will eventually conduct thereby degrading the regulator's output voltage stability.
It is an object of the present invention to provide a start-up circuit for a band-gap regulator that is reliable over a large operating temperature range.